1. Field of the Invention
The present invention relates to a semiconductor device having a diaphragm, which can be used as a semiconductor pressure detection device which detects pressure by using the diaphragm, and a method for producing the semiconductor device.
2. Related Art
A sectional view and a plan view of a conventional semiconductor pressure sensor are shown in FIGS. 12A and 12B respectively. As shown in FIG. 12A, the semiconductor pressure sensor includes P- type silicon wafer 1 having a diaphragm C formed by removing a part of the P- type silicon wafer 1, and an N- type epitaxial layer 2. As shown in FIG. 12B, P+ type diffusion gauge resistors (piezoresistances) 3 corresponding to a bridge circuit are formed over the diaphragm C. Aluminum wirings 4a-4d connected with their outer terminals are formed at edges of the gauge resistors 3. The aluminum wiring 4a is connected with an N+ type diffusion layer 6 which is formed on the surface of the N- type epitaxial layer 2, and also connected with a power terminal 5a of the bridge circuit. The aluminum wiring 4d is connected with a ground terminal 5d of the bridge circuit. In operation, current flows from the power terminal of the bridge circuit through the aluminum wiring 4a, and electric potential is supplied to the gauge resistors 3. When the diaphragm C is displaced by pressure and the gauge resistors 3 are stretched or contracted, the resistance of the gauge resistors 3 is changed. Thus, the electric potential at the mid-point of the bridge circuit is changed. The pressure is detected based on the electric potential of the mid-point.
The electric potential of the N- type epitaxial layer 2 is fixed to a predetermined voltage through the N+ type diffusion layer 6. The electric potential of the N- type epitaxial layer 2 is determined so that the electric potential of the N+ type diffusion layer 6 and the N- type epitaxial layer 2 are greater than or equal to the electric potential of the gauge resistors 3. The electric potential of gauge resistors 3 is greater than or equal to ground (0 electric potential). As a result, a forward bias is not provided between the N- type epitaxial layer 2 and the gauge resistors 3.
However, in the above-described semiconductor pressure sensor, because a small leakage current actually flows between the N- type epitaxial layer 2 and the P- type silicon substrate 1, a potential difference may exist across the N- type epitaxial layer 2. Therefore, the above-described relation of the electric potential of the N+ type diffusion layer 6, the N- type epitaxial layer 2 and the gauge resistors 3 may not be maintained.
The potential distribution of the conventional semiconductor pressure sensor is shown in FIGS. 13A and 13B. The electric potential of the N- type epitaxial layer 2 near the gauge resistors 3 may be less than that of the gauge resistors 3. As a result, the pn junction between the N- type epitaxial layer 2 and the gauge resistors 3 becomes forward biased, and a larger leakage current flows from the gauge resistors 3 to the N- type epitaxial layer 2. As a result, the current which flows in the gauge resistors 3 can change, and it becomes impossible to accurately detect pressure.